The document SAMFLO FR 2 751 675 discloses a method of construction of a vault, in which two half-shells, provided with a lip and bearing braces, are positioned on continuous footings constructed on site. The two half-shells are then brought to bear against one another, by means of the braces, and a continuity keystone is cast between the upper edges of the two half-shells, on the lips, so as to construct a keystone which brings about continuous transverse joining together of the half-shells.
The braces, by means of which the half-shells bear against one another before the keystone is formed, are embedded in the concrete and are thus lost. However, these braces make up a certain percentage of the cost of construction of a vault. The applicant has therefore sought to reduce the cost induced by these braces.
From FR 2 723 116 (PREFAC), a method of construction of a vault using at least two half-shells each comprising a bottom lip was known, in which the two half-shells are brought to bear against one another, lip against lip, with a compressible block being interposed, and, in order to avoid any relative escape of the two half-shells and thus achieve secure bearing, the two lips are connected by means of a metal plate which is fixed to the lower faces of the lips. The plate is positioned after the block and, consequently, after the compression of the latter and after the half-shells have been brought to bear against one another. The result is that the plate does not participate in the half-shells being brought to bear against one another. Concrete is then cast onto the bottom lips, between the two half-shells, so as to produce a longitudinal keystone beam.
In the PREFAC invention, the block and the plate are normally removable. However, as the block is compressed between the two half-shells, it proves impossible to remove it later. Moreover, if the block is interposed between the lips, it is the latter which support the bearing load of the two half-shells on one another, even after the keystone concrete has been cast, and not the longitudinal keystone beam. The result is that the vault constructed in this manner is liable to premature wear.
In the SAMFLO invention (FR 2 751 675), there is no transfer of the bearing load from the braces to the keystone either. Indeed, it is by means of the braces, embedded in the concrete, that the half-shells bear against one another not only before the concrete is cast but after casting also.